I wrote this stupidly long post in the big spoiler thread and realized it doesn't really relate to the big spoiler.
So I thought I'd share, it's about the Tarot cards Solas gets (The Tower & The Heirophant)
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I think that the fact that when romanced it changes to the Heirophant is a way to give those who endured that horribly sad romance a sneak peak to his personality? In a good way. Romaced Lavellan knows Solas the most, she has the best idea of who he is and his motives (still not a lot of idea, but still...), I think it's a way to expand on that knowledge to the player.
Also, we should consider the meanings of the cards.
The Tower - this one comes to describe the fact that "omg Solas is Fen'Harel' as well as what Solas is trying to achieve. The Tower speaks of (usually disastrous) realization/revelation, when what you think is true is revealed to be false, it also speaks of sudden change. So - the revelation that Solas is Fen'Harel (obviously) and that he has been hiding the truth of his connection to Cory all along, as well as his plan to bring about a big change.
Classic Tower (from a deck I own):

The Heirophant - this is trickier. This card revolves around religion and beliefs, as well as a cause. He is also known as the Pope. It can also speak of "doing the right things". This card speaks of status quo, it's the exact opposite of the 'change' in the Tower. It's quite bewildering since it's obvious Solas is still up to executing his plans. The question is - does this hint that he changed some of his opinions?
On the other hand it can speak of another aspect of the card, spirituality, mercy/goodness, consciousness, connecting "god" and the people sort of thing. As well as striving for harmony within a community, maybe this comes to explain his final goal?
Classic Heirophant (this is a deck I own):

Also, this card has a female counterpart - the High Priestess, I kind of like thinking of Lavellan as her.
Please notice that there aren't really bad cards in Tarot, they can be bad in certain context as well as good. There is a Devil card (generally pleasure/indulgence/desire etc.) and Death card (generally endings/big change).
OK, an addition.
You all probably have noticed how well Solas romance card & Lavellan's tarot card go together (they are facing each other arms extend towards each other, both holding a plant in their hands. Both holding a staff (which is odd, the Dalish cards are the only ones to hold weapons).
I've been thinking that maybe she is the High Priestess.
There are 2 obviously female tarot cards in the major arcana - Empress & High Priestess and their equivalents are the Emperor & Heirophant cards. There are also several others which are gender neutral in meaning but usually depict a female - Star, World & Strength, but I won't speak about them.
The Empress card - represents fertility, femininity, beauty, abundance. While I can go and try to figure out this card, I believe what she stands for doesn't really reflect the Inquisitor, this card is rather shallow.
The High Priestess card - basically represents intuition, mystery, subconsciousness (dreams?), she stands for gaining knowledge before making decisions. The Dalish female card also has an aura of mystery, unlike Solas' card which appears more upfront (I suppose?). The other 3 female cards are a lot more assertive, the Dalish female card is... quiet. A romanced Lavellan needs to be inquisitive, seeks knowledge and she be open to ideas and mysteries (otherwise she'll have a hard time making Solas interested, I'm sure it is possible romancing him and not being all of these, but it's probably really hard).

I'm pretty certain of it, I'm not sure there are specific cards for the other race/gender combo, but there could be (we just need to figure out the clues), but since female Lavellan is the only one who can romance Solas (who is the Heirophant) then it fits really neatly.
Even too neatly.
I know Vivienne is the high priestess (which suits her, sort of, it's more of her front than her real self) but since the cards for the races/gender do not appear in the tarot deck I don't really think it matters. Plus, have you noticed how much Vivienne's card & fem!Dalish card have in common?

Facing right, heads in profile, similar torso and lower body positions (torso leaning back, left leg in front), staves in their left hands, objects in their right hands, colors matching.
Probably reading too much into this.
I just like the idea that, since they didn't give us a lot of romance content, that they left clues for us.